OpenAI Codex Essentials

Codex is OpenAI's coding agent. It comes in a few forms: a CLI that runs locally in your terminal, IDE extensions (VS Code, Cursor, Windsurf), a desktop app (codex app), and Codex cloud, the same agent running in your browser at chatgpt.com/codex.

Setup

Install the CLI on macOS or Linux:

curl -fsSL https://chatgpt.com/codex/install.sh | sh

Or via npm or Homebrew:

npm install -g @openai/codex
brew install --cask codex

Then run codex and select Sign in with ChatGPT. Codex is included with ChatGPT Plus, Pro, Business, Edu, and Enterprise plans. You can also use an API key with additional setup. If you're picking a daily driver and you already pay for ChatGPT, this is the zero-cost option.

The core loop

Start codex in a project and describe a task in plain language. Codex reads the repository, proposes changes, and (subject to your approval settings), edits files and runs commands to verify.

cd your-project
codex
# > Fix the flaky auth test and run the suite

Approval modes and sandboxing

Codex lets you control how much autonomy it has: this is a genuinely useful feature for interviews, because it forces you to consciously decide what the AI may do without you.

Interview transfer: in an interview you are the approval layer. If you're in the habit of letting the tool auto-approve, practice consciously deciding at each step. That's the "stay in control" skill, trained by your own tooling.

AGENTS.md

Codex reads an AGENTS.md file in the project root for persistent instructions: coding standards, commands, conventions. Many agents now share this convention (OpenCode generates one via /init, and other tools read it too), so committing a good AGENTS.md to a project is the most portable way to make any AI tool behave consistently.

Interview transfer: same as CLAUDE.md, if your open-ended practice repo ships with an AGENTS.md that lists build/test commands and conventions, the AI stays grounded in your environment.

Codex cloud

Codex cloud (chatgpt.com/codex) runs the same agent in the browser: no local setup, works on repos you don't have locally, and lets you run long tasks in parallel. For interview practice this is a decent stand-in for a structured platform: the agent has repo access but lives outside your editor.

Interview-relevant habits